1. Field of the Invention
The present invention relates to a focusing technique.
2. Description of the Related Art
There are various automatic focusing (hereinafter also referred to as AF) control methods for image pickup apparatuses such as video cameras. In a so-called TV-AF method, a predetermined high frequency component (contrast component) of an image signal is extracted, and a focusing lens is controlled so that an AF signal corresponding to the sharpness of the image is largest. In another method, information corresponding to the distance to the object is obtained by measuring the phase difference of the object or by triangulation. A combination of these methods (hybrid AF control method) is proposed in Japanese Patent Laid-Open No. 2002-258147.
The hybrid AF control method is a combination of the TV-AF method and an internal measurement method or an external measurement method. In the internal measurement method, light incident on an image pickup optical system is split and input into a sensor that measures the phase difference, and the amount of deviation between the focal position and the in-focus position is measured (the first embodiment of Japanese Patent Laid-Open No. 2002-258147). In the external measurement method, a sensor that measures the phase difference is provided separately from an image pickup optical system (the third embodiment of Japanese Patent Laid-Open No. 2002-258147).
In the internal measurement method, the same object image as the object image picked up in the image pickup optical system is input into the sensor that measures the phase difference. Therefore, the sensor can surely capture the object. However, since a light splitting mechanism and the sensor are placed in the lens barrel, the size of the lens barrel is liable to be large and therefore the size of the image pickup apparatus main body is liable to be large.
In the external measurement method, since the sensor that measures the phase difference is disposed separately from the image pickup optical system, there is a lot of flexibility of layout. Therefore, this method is advantageous to the size reduction of the apparatus. However, a deviation occurs between the distance to the main object to be photographed and the distance to the object measured by the sensor that measures the phase difference. Focusing can fail or take a long time. To solve this problem, for example, Japanese Patent Laid-Open No. 6-90395 proposes a hybrid AF control method in which AF methods are switched based on the comparison between the distance measured by a ranging sensor and a predetermined distance.
However, in the above-described external measurement method, since the distance sensor is disposed independently from the image pickup optical system, a deviation occurs between the optical axis of the image pickup optical system and the measurement axis of the distance sensor, that is to say, a parallax occurs. FIG. 2 shows the photographing range and the measuring range of an external sensor that measures the phase difference in the case where a hand is held in front of the image pickup optical system of an image pickup apparatus. In the case where a hand is held in front of the image pickup optical system, as shown in FIG. 2, the object photographed through the image pickup optical system (the hand) is different from the object A captured by the external sensor. In this case, if the focusing operation is started based on the measurement result of the external sensor, a malfunction of the hybrid AF control is induced. For example, focusing on the object of the image pickup optical system fails or takes a long time.